The "OS" isn't special here, apps can listen to system audio.
fwiw, you only need to know anything about outputs if you are doing AEC. Blind source separation doesn't have that problem and can just process the input stream.
> The "OS" isn't special here, apps can listen to system audio.
Even if this is true, it's easy to imagine such functionality being exploited by malicious apps as a security and/or privacy concern, particularly if the user needs a screen reader.
It definitely makes sense for the operating system to provide this functionality.
fwiw, you only need to know anything about outputs if you are doing AEC. Blind source separation doesn't have that problem and can just process the input stream.